1. Field of the Invention
The present invention relates to apparatus and methods pertaining to ball grid array technology and, more specifically, to apparatus and methods for supplying solder balls.
2. Description of the Related Art
The electronics industry is one of the most dynamic and important industries today. It has literally transformed the world and provides many products that affect our daily lives, for example, telephones, television, personal computers, cellular phones, pagers, video camcorders, audiovisual products, etc. A key area of development in the electronics industry is the area of chip level packaging and interconnections. Solder bumping provides the highest level of packaging density with the least package space. The solder bumping is created by solder balls which are re-flowed onto connection points or pads on the chip and/or the package. The solder balls are arranged in arrays on the chips and the packages. These arrays are known as ball grid arrays ("BGA").
Ball grid array packaging is rapidly emerging as the technology of choice for high input/output (I/O) count integrated circuits (IC's). Ball grid arrays deliver higher density and yields than traditional packages without requiring fine-pitch processing or new assembly equipment. Driven by the increasing I/O as IC's become larger and more complex, the demand for ball grid array packages is expected to grow from fewer than 20 million units in 1995 to more than 2 billion by 1999.
The low cost, high volume nature of the electronics industry requires highly automated production. Solder balls must be placed on chips at extremely high rates automatically. Equipment for performing operations involving solder balls has become very quick and efficient. To be cost efficient, the industry requires a long mean time between assistance (MTBA) of the process by an operator. The main factor which dictates the MTBA for ball grid array equipment is the solder ball capacity of the equipment. Typical BGA equipment have reservoirs which will hold on the order of 250 thousand solder balls. The actual number of solder balls is dependent on the size of the solder ball being processed. Typical BGA equipment requirements are MTBA's of 4 hours, with the industry pushing for increases to 6 and even 8 hours. Additionally, BGA equipment manufacturers continue to improve in throughput rate (the number of BGA's per hour produced by the equipment). As a result, solder ball reservoir capacities must increase to maintain the same MTBA.
The solder balls typically used in BGA technologies generally comprise eutectic solders such as tin-lead solders, solder coated copper, or high temperature alloys. Examples of tin-lead solder ball compositions are 63% tin--37% lead and 62% tin--36% lead--2% silver, typically for use with plastic ball grid arrays, and 10% tin--90% lead, typically for use with ceramic ball grid arrays, although other compositions are possible. The solder balls used in current applications typically have sizes ranging from as small as 5 mills (thousandths of an inch) to as large as 30 mills. Other sizes and size ranges may be used as well. As used herein, the term "solder ball" is used to refer to the generally spherical unit of solder used in ball grid arrays to bond and electrically couple electrical connections such as solder pads or bonding sites on substrates such as chips to an object to which the substrate is to be electrically coupled.
It is generally a necessity for high-quality ball grid array technology that the solder balls be accurately placed on the solder pads. Placement must be done in a way that preserves the physical and geometric integrity of the solder ball. It is generally unacceptable, for example, for any deformation of the solder balls, slicing of the solder balls, etc. to occur. When a solder ball is cut in two during handling or processing, for example, it is generally necessary to stop the process, inspect the solder ball reservoir for ball fragments, and to remove those ball fragments before processing can continue.
There are a large variety of methods and apparatus for distributing the solder balls onto substrates, and otherwise for the handling, control and placement of the solder balls. U.S. Pat. No. 5,499,487, issued to McGill on Mar. 19, 1996 and U.S. Pat. No. 5,551,216, issued to McGill on Sep. 3, 1996, provide examples. In these patents, a device is disclosed which comprises an apparatus for placing solder balls in a ball grid array. BGA equipment, such as is disclosed in these patents, have limited reservoirs for supplying solder balls to the equipment. Due to the design of the BGA equipment, the equipment reservoirs are exposed to the atmosphere. Solder balls are inherently sensitive to corrosion when exposed to the atmosphere. Solder ball exposure time to the atmosphere must be limited to prevent damage to the solder balls. The BGA equipment solder ball reservoirs are small and must be refilled frequently, decreasing the MTBA of the equipment. It is desirable to automatically refill the reservoirs on the equipment so operator assistance is not required. Large reservoirs automatically feeding the equipment reservoir have been used to increase MTBA's. These automated reservoirs, or solder ball supplies, are typically sealed to minimize atmospheric exposure of the solder balls. Prior solder ball supplies have typically used gravity to supply the solder balls to the equipment reservoir and mechanical gates to start and stop the flow of solder balls to the equipment.
One limitation of prior solder ball supplies is that the mechanical gates generally used have a tendency to damage some of the solder balls. These mechanical gates can also have a tendency to cause solder balls to clog and jam the flow of solder balls at the gate. This is due to the discontinuous nature of the flow path at the location of the gate, which is generally required by the use of mechanical gates. Another limitation of prior solder ball supplies is that they generally do not allow for the adjustment of solder ball supply rates. Another limitation of prior solder ball supplies is that they generally have limited solder ball capacities. Another limitation of prior solder ball supplies is that they generally do not provide for quick changeover of solder balls from one type to another.